Single-molecule magnets have potential data-storage applications, but will need to work at a much higher temperature than has been possible. Two studies suggest that this goal could be met in the near future. See Letter p.439
More than two decades ago, it was discovered that a single molecule could have two stable magnetic states — a property called magnetic bistability — at temperatures of a few kelvin1. In principle, such molecules could be used to store information, but this would require operating temperatures approaching at least that of liquid nitrogen (about 77 K). On page 439 and writing in Angewandte Chemie, respectively, Goodwin et al.2 and Guo et al.3 report a remarkable advance in this direction. The authors demonstrate magnetic bistability in a molecule at temperatures as high as 60 K, which is twice that achieved in previous studies4.
Single-molecule magnets are compounds consisting of a metal ion and organic ligands that exhibit magnetic bistability below a certain 'blocking' temperature. This temperature is mainly determined by the energy required to switch between two opposite orientations of the molecule's magnetic moment5. A key factor is the metal ion's magnetic anisotropy — the extent to which the ion's response to a magnetic field depends on the direction of the field. Ions of lanthanide elements can have relatively large magnetic anisotropies and are therefore well suited to achieving high-temperature magnetic bistability. However, single-molecule magnets that contain such ions need to be carefully designed to optimize the spatial distribution of the ligands' electrons with respect to that of the ion's electrons.
In the case of lanthanides whose electron distributions are oblate (squashed along the axial direction), such as terbium, dysprosium and holmium, an axial position of the ligands' electrons is particularly favourable for generating a sizeable magnetic anisotropy. However, lanthanide ions are often surrounded by many coordinated (neighbouring) ligand atoms — in general, 8 to 10 — that are approximately spherically distributed around the ion.
A strictly axial coordination environment can be realized, for instance, by depositing a holmium atom on top of an oxygen atom in a thin layer of magnesium oxide6. In such a system, magnetic bistability has been observed for temperatures as high as 30 K, allowing the holmium atom's magnetic moment to be detected and manipulated using scanning-probe-microscopy techniques7. Although such atomic nanostructures have retained the record blocking temperature for about a year6, it now seems that molecules are back in the race to achieve liquid-nitrogen temperature.
Goodwin et al. and Guo et al. generate an axial coordination environment by sandwiching a dysprosium ion (Dy3+) between two five-membered carbon rings called cyclopentadienyl anions (Cp−) (Fig. 1). Each ring has three t-butyl [C(CH3)3] substituents attached, which hinder the binding of other ligands. Despite this hindrance, the Dy3+ ion can accommodate a chloride ion (Cl−) in an equatorial position, creating a neutral [(Cp)2DyCl] complex. The extra ligand strongly reduces the magnetic anisotropy of the Dy3+ ion and breaks the symmetry of the ligand charges around the metal ion. The latter is crucial for suppressing relaxation processes5 that reduce the molecule's remnant magnetization in the absence of a magnetic field.
The authors show that, by chemically removing the Cl− ligand, the magnetic properties of the molecule change. Whereas the magnetic moment of [(Cp)2DyCl] is free to fluctuate, the charged [(Cp)2Dy]+ complex, which forms molecular crystals with a negatively charged organic ion, has remarkably robust magnetic bistability at temperatures as high as 60 K. The key to the success of both research groups is the use of strongly hindered Cp− ligands, which reduces the instability of the [(Cp)2Dy]+ complex. Goodwin and colleagues demonstrate that the magnetic properties are purely molecular in origin by dispersing the complex in crystalline or glass-like matrices and showing that there is no major alteration in the magnetization dynamics.
The design strategy used by Goodwin et al. and Guo et al. has been known8 since 2011, and has been used extensively by scientists. What makes [(Cp)2Dy]+ so special that it can reach such a high blocking temperature? Goodwin and colleagues provide a tentative explanation by performing state-of-the art ab initio modelling of the complex's magnetization dynamics. Their analysis suggests that the coupling of the molecule's magnetic moment with vibrations of atoms inside the molecule and of molecules inside the crystal lattice is responsible for the observed properties. Earlier this year, it was demonstrated that low-energy vibrations, in particular those involving metal ions and coordinated ligand atoms9, can be extremely detrimental to magnetic bistability. The fact that the Dy3+ ion in [(Cp)2Dy]+ is coordinated only by carbon atoms, which are rigidly bound in Cp− rings, seems to reduce the effect of these vibrations.
Can we reasonably expect further progress in this field? Merging the top-down approach of atomic nanostructures on surfaces with the bottom-up approach of molecules is a potential way forward. Combining molecules that have a relatively high magnetic anisotropy with stiff substrates has already been found to strongly enhance magnetic bistability — at least when the latter is investigated in ultrahigh-vacuum conditions, as has been the case for the archetypal lanthanide single-molecule magnet terbium(III) bis(phthalocyanine)10. Unfortunately, the molecular system investigated by Goodwin et al. and Guo et al. is not well suited to be processed in this way, given its positive charge and high reactivity.
Strategies to improve magnetic bistability could take advantage of theoretical investigations that have shown, for instance, that low-energy molecular rotations can also be coupled to the molecule's spin11. The frequency of these rotations is inversely proportional to the molecule's mass. Therefore, smaller molecules would be less affected by this mechanism than larger ones because higher temperatures would be required to activate the rotations. Consequently, smaller molecules would show enhanced magnetic bistability. Finally, embedding the molecular complex in a rigid network, such as a metal–organic framework, is an alternative strategy for pushing single-molecule magnets further towards data-storage applications.Footnote 1
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